The invention relates to a method for operating a water softening system having an ion exchange device, comprising: an ion exchange resin, a supply vessel for supplying a regenerating solution for regenerating the ion exchange resin, an admixing device, and at least one flow meter,
wherein an influent volume flow V(t)raw of raw water towards the water softening system is divided into a first partial volume flow and a second partial volume flow upstream of or within the water softening system, with the first partial volume flow being guided through the ion exchange resin and that softened partial volume flow V(t)partial1soft being admixed to the second, raw water-bearing partial volume flow V(t)partial2raw, whereby in or downstream of the water softening system an effluent volume flow V(t)admixed of admixed water is formed, wherein the ratio between the first and second partial flow in the effluent volume flow V(t)admixed of the admixed water may be adjusted via the admixing device,
wherein the method comprises the following steps:
                determining the conductivity of the raw water by means of a conductivity sensor, and determining therefrom the total hardness of the raw water using a calibration characteristic which is stored in a control unit,        determining the first partial volume V(t)partia1soft directly or indirectly using the at least one flow meter.        
A method of this kind is known from EP 0 900 765 B1.
If there is a requirement for softened or partially softened water due to technical reasons or reasons of convenience, water softening systems are used in different applications. In the softening according to the ion exchange method the hardness-forming ions, calcium and magnesium ions, are replaced with sodium ions. This is done by means of a resin (ion exchange resin) loaded with sodium ions. If the resin is depleted it has to be regenerated with salt brine, so as to be available for the softening process again.
The point in time when the ion exchange resin has matured to depletion and regeneration is imminent depends on the nominal capacity of the water softening system, on the water quality (i.e., the quality of the raw water), and on the water consumption. The major problem is to determine automatically and positively the appropriate point in time of regeneration independently of the water quality. if regeneration is started prematurely there will be a rise in salt consumption and thus the environment will be polluted, and if regeneration is started too late, there will be a hardness breakthrough.
One method to determine the point in time of regeneration of a water softening apparatus is known e.g., from DE 198 41 568 A1. Here, the electrical conductivity is measured upstream and downstream of the softening device and a difference of conductivity is ascertained therefrom. The disadvantage of this method, however, is that in the calcium/sodium exchange the conductivity undergoes minor changes only and, as a consequence, the point in time of regeneration cannot be determined exactly. Also, regeneration can only be initiated if an incomplete softening has already occurred, i.e., a hardness breakthrough has commenced.
Document U.S. Pat. No. 6,814,872 B2 describes an apparatus and a method for controlling regeneration of a water softener. For the purpose, the water hardness and the volume of the water to be softened are determined by means of a conductivity sensor or an ion-selective electrode and a water meter and the signals are outputted to an electronic controller. The point in time of regeneration is determined by making a comparison between a total level of hardness removed from the water so far and a water softener capacity. Ion-selective electrodes, however, are failure-prone and require regular maintenance efforts. Further, such electrodes are expensive.
Technical or economical reasons often make it necessary or desirous to only use partially softened water. It may occur that water which has been softened completely gives rise to corrosion problems whenever the formation of a protective coating is no longer possible in the downstream piping installation. Further, the capacity of the softener is rapidly depleted in the case of a complete softening, so early regeneration is necessary. This goes along with a high consumption of salt and does entail high expenditure. To implement partial softening, a device (admixing device) for the mixing of softened water (also known as pure water or soft water) and raw water is required. As a rule there is a desire for adjusting and controlling the water hardness in the admixed water, i.e., the admixture of softened water and raw water.
A device for water softening in accordance with the ion exchange method using an admixing device is described in EP 0 900 765 B1. For controlling the regenerating process of the ion exchanger and for admixing raw and pure water, the hardness of the influent raw water is determined by means of conductance sensors. Further, flow meters for pure and raw water plus additional conductance sensors are used in the ion exchange material, the signals of which are processed by an electronic evaluation and control unit. However, this known device has a relatively complex configuration due to the large number of electrodes required and thus is expensive, Moreover, adjusting the hardness of the admixed water using this device is rather inexact.